Preheat oven to 350 degrees F (180 degrees C).
Chop Cauliflower (1 head) into florets.
Steam cauliflower florets over about five centimeters of simmering water. Steaming should take about eight minutes. Make the cheese sauce in the meantime.
Melt Salted Butter (3 1/2 tablespoon) in a saucepan over medium heat.
Add All-Purpose Flour (50 gram) and whisk to combine.
Reduce heat to low and allow mixture to cook for about two minutes, or until thickened. Keep stirring throughout to prevent from burning.
Gradually add in the Milk (500 milliliter), continuing to whisk.
Continue to whisk and cook until the mixture thickens, about ten minutes.
Put cauliflower florets into a baking dish and cover with the cheese sauce. Season with Salt (to taste) and Ground Black Pepper (to taste).
Add additional Mature Cheddar Cheese (75 gram) on top.
Bake for 25 minutes at 350 degrees F (180 degrees C), or until cheese is bubbling and golden brown.